There is a correlation between damage to the surface of a rolling bearing such as a ball bearing and the oil film parameter (.LAMBDA.=h/.sigma.), wherein h is the thickness of an oil film at a rolling contact portion, and .sigma. is the composite roughness. That is, the smaller the oil film parameter .LAMBDA., the more the rolling elements of the rolling bearing are likely to come into direct contact with each other and the more they are subject to surface damage. The life of the bearing thus shortens.
If the oil film parameter .LAMBDA. is sufficiently large, an oil film is formed between rolling elements and direct contact is prevented, so that the rolling bearing will enjoy a long life.
Some conventional rolling bearings have the surfaces of their rolling elements mirror-finished by machining to minimize any surface damage and thus to prolong their lives.
Other conventional rolling bearings have microscopic recesses in the form of pits in their surfaces to increase the thickness of oil films formed between the rolling elements. For the same purpose, it is also known to form discontinuous grooves extending in a direction substantially perpendicular to the rolling direction of each rolling element.
An increasingly large number of today's rolling bearings are used in a low-viscosity oil or at high temperatures in order to minimize the energy loss due to friction between machine parts equipped with the rolling bearings and thus improve the machine's performance.
The lower the viscosity of the lubricating oil and/or the higher the operating temperature, the thinner the oil film thickness h tends to be, and the more the rolling elements are likely to come into direct contact with each other. Under operating conditions where the oil film parameter .LAMBDA. is less than one, rolling elements will be in direct contact with each other at all times.
Lubricating conditions where rolling elements are brought into direct contact with each other are generally called "boundary lubrication" or "mixed lubrication".
Today's rolling bearings have surfaces with such little roughness that these surfaces can be practically called mirror-finished surfaces. It is impossible to prolong the life of such bearings used under boundary lubricating conditions by the conventional method in which the oil film parameter .LAMBDA. is increased by adjusting the surface roughness to prevent direct contact between rolling elements.
It became apparent from recent studies that there is a possibility that oil films may be formed in microscopic areas on a contact surface under boundary lubricating conditions if the contact surface has microscopic protrusions or recesses. Formation of oil films in such microscopic areas is called "micro-EHL effect".
One conventional ball bearing is formed with pits in its rolling surfaces to achieve the micro-EHL effect. This ball bearing has however a problem in that, due to a large depth of the pits, the acoustic value tends to be high as compared with a bearing having finished surfaces with no pits.
Although it is possible to reduce the depth of such pits by barreling, and thus reduce the acoustic value, such barreling will reduce the number of pits, thus reducing the thickness of the oil films. That is, such barreling impairs the otherwise excellent oil forming ability of the pits.
The method of forming discontinuous grooves perpendicular to the rolling direction of a rolling element is applicable only if the rolling element is of a type whose rolling direction never changes, such as rollers of roller bearings. This method is not effective for rolling elements whose rolling direction is not fixed but varies, such as balls of a ball bearing.
An object of the present invention is to provide rolling elements for a ball bearing which can avoid direct contact between the rolling elements even if the bearing is used under boundary lubricating conditions.
According to the present invention, there is provided a ball for a ball bearing having a surface formed with a multitude of microscopic needle-shaped recesses extending in random directions and present in a dispersed manner. The recesses measure 2.0-30 .mu.m in length and 0.3-2.0 .mu.m in width. Per 100-square-micrometer area of the surface, one to fifteen (1-15) recesses are present. The centerline average roughness of the ball should be from 0.005 .mu.m to 0.012 .mu.m.